Malignant tumors of the immunoglobulin (Ig) forming system provide a series of models in which to study a variety of biologic processes. The availability of amino acid sequence analyses allows correlation of data concerning Ig synthesis, assembly and secretion with precise chemical features. The tumor cells can be adapted to tissue culture and clones typical of the entire population isolated and studied. Clones which vary from the parent with respect to any of a number of biologic properties (e.g. Ig synthesis) can be isolated (with or without mutagenesis) and the genetic and/or epigenetic factors controlling Ig synthesis can be investigated. The tumors described here share certain properties, e.g. IgM synthesis, yet vary one from another. This variation appears to be systematic and may reflect a spectrum of differentiated states, each seen only transiently, in the normal course of the somatic cell development of Ig producing cells. Hence the process of "B-cell" ontogenesis may be experimentally approachable. Finally all these tumors were originally induced in the same way, i.e. by intraperitoneal injections of Freund's adjuvant or mineral oil. Yet, they appear to vary in the expression of the intracisternal A- particles which have been associated with a number of murine tumors. Investigation of the mechanism of this variability of expression with respect to the differentiated state of the host B-cell may yield some insight into the relationship between these particles, immunodifferentiation and malignancy.